The present application relates to methods for joining superconducting wires together, and joints as may be made by such methods.
When manufacturing equipment such as electromagnets from superconducting wire, it is commonly required to join separate lengths of wire together. In order to maintain the superconductivity of the equipment, the joints must also be superconducting, or at least exhibit very low resistance, if operation in ‘persistent-mode’ is required. Typically, joint resistances of ˜10−13 ohms are required to enable this mode of operation. Operation in ‘persistent mode’ is highly desirable as this enables the power supply to be dispensed with after initial energization has been achieved.
Recent developments in superconducting materials have led to the use of magnesium diboride MgB2 as a superconducting material. Magnesium diboride MgB2 has the benefit of exhibiting superconductivity at higher temperatures than more conventional materials, avoiding the need to cool the superconductor to very low temperatures. However, the material itself is brittle, and difficult to join to form persistent joints.
FIG. 1 shows a cut-away view of a typical MgB2-core superconducting conductor 10. Superconducting filaments 4 comprise an MgB2 core 1 in an essentially granular, powder form, held within sheaths 2 of niobium metal. These MgB2-filled niobium sheaths are further encased in a matrix 3 of high strength, conductive metal or alloy, such as the Cu—Ni alloy known as “MONEL”. The matrix 3 and filaments 4 make up superconducting wire 7. The purpose of the niobium 2 is to prevent unwanted reactions occurring between the MgB2 and matrix material during wire manufacture.
In one manufacturing method, known as the ex-situ process, granulated or powdered MgB2 is placed in a number of niobium lined holes drilled into a billet of matrix material. The complete billet is then drawn to the required final wire diameter. The Niobium-cased superconducting filaments are formed and compacted during the drawing process.
The matrix 3 provides an electrically conductive shunt and thermal sink. Should any of the superconducting filaments 4 quench, then heat will be carried away from the quenched region by the matrix 3, and electric current will flow through the lower resistance offered by the matrix. This will allow the quenched part of the filament to cool back to superconducting condition. The matrix also makes the superconducting wire more mechanically robust.
The conductor 10 typically also comprises a stabilizing channel 5. This may be of copper or another material, or combination of materials. The channels should be electrically and thermally conductive. In the illustrated example, the wire 7 is soldered at 6 into a cavity of the channel 5. The channel 5 adds further electrical and thermal stability, and mechanical robustness, to the superconducting wire 7, in the same manner as explained with reference to matrix 3.
In order to make a superconducting joint, two conventional approaches have been adopted: firstly, a joint may be formed directly between the MgB2 cores 1 of the wires to be joined. Alternatively, another material, which is also superconducting at the temperature of operation of the wire, is used to electrically join the MgB2 cores 1 of the wires together in a superconducting arrangement. Typically, known joining methods involve exposing the MgB2 cores of the superconducting wires to be joined, and mechanically pressing the exposed MgB2 particles of the respective wires together to form the superconducting joint. In some known arrangements, an intermediate layer of a superconducting material, typically a metal such as indium is interposed between the exposed cores of the respective wires, to increase the contact surface area and improve mechanical adhesion between the particles of the respective wires. Such methods require significant mechanical loads to be applied to the MgB2 particles. The MgB2 particles are relatively brittle, and applying such significant mechanical loads risks fracturing the MgB2 superconducting material, leading to failures of the superconducting joint.
In some known methods, MgB2 particles are exposed and heated, for example when joined by MgB2 powder or a reaction between magnesium and boron powders. If the MgB2 particles are exposed, there is a risk of oxidation. Failures may occur sometime after the jointing process, after the joint is built in to a superconducting device, such as a magnet within a cryogen vessel. Such failures are very expensive and time-consuming to repair, due to the access problems of reaching a joint within a superconducting device built into a cryogen vessel, and/or vacuum vessel, and so on. It is therefore an object to provide methods for joining MgB2-cored superconducting wires which reduce the risk of mechanical damage, or oxidation, to the MgB2 particles.
However, tests on conventional joints between MgB2-based superconducting wires have shown magnetic field tolerance values poorer than expected. This is believed to be due to conduction actually taking place through the niobium of the sheaths 2 rather than through the superconducting joints between MgB2 particles of the respective wires. Niobium is a “Type II” superconductor, but has a very low upper critical magnetic field strength Bc2 when compared to other Type II superconductors such as the alloy niobium titanium. The critical field of niobium is in the range of a few tenths of a tesla with exact value depending on many factors, most notably the current density. Since it is highly desirable that joints for use in superconducting magnets should be able to tolerate quite high magnetic fields, any jointing method that utilizes the niobium sheaths for current transport is likely to be of little use.
Certain conventional methods for producing superconducting joints are described in WO2007/128635A1, US2008/0236869A1, U.S. Pat. No. 6,921,865B2 and U.S. Pat. No. 7,152,302B2.